Summary Intracranial Aneurism (IA) is a disease in which weakened blood vessels in the brain bulge and fill with blood, often leading to rupture, brain hemorrhage and sudden death. Strikingly, people with intracranial aneurism don't manifest obvious symptoms even minutes before hemorrhage, making preventative diagnosis and treatment obsolete. Up to 10% of the general population has a high probability of carrying a brain aneurism, revealing a serious burden to public health. Every year, 30,000 people in the U.S. alone experience brain hemorrhages that cause death or brain damage requiring long-term care. Since the exact cause(s) of brain aneurism remains elusive, the only hope for these patients is to first, identify risk factors predictive of the vascular lesion in the brain and second, find a strategy to prevent the bleeding events. Although genetic predispositions for aneurisms has been discovered in multiple families, whether the mutations associated with IA directly cause the disease is unknown. The objective of our proposal is to understand the cellular and molecular events that lead to brain vessel fragility, with the long-term goal of uncovering new approaches to treat and prevent IA. To determine whether candidate mutations cause IA and uncover their pathological mechanisms, we will utilize a powerful zebrafish model system that uniquely recapitulates the loss of function of the PPIL4 gene, which variants are strongly associated with IA. We plan to apply sophisticated cellular and genetic manipulation of our zebrafish model to discover how PPIL4 mutations affect Notch signaling to trigger vessel rupture, and how to prevent this outcome. Specifically, we propose to study how PPIL4 controls the interaction between neurovascular cells (Aim 1), how PPIL4 functions in healthy fish to promote the stability of the Notch intracellular domain (Aim 2) and examine if genetic and chemical strategies can prevent vascular leakage in the ppil4+/- model (Aim 3). The outcome of this study will help to elucidate the requirement of this novel gene in the neurovascular system and to understand how human PPIL4 mutations may cause vascular defects and susceptibility to IA.